Use models as playlists

this is a proof-of-concept, using the listStore of the Album view as a
playlist if one item is clicked.
Various enhacements can be made (embedding model/iter in one object,
warning that the song as been paused...) and the theory remains open to
discussion
fix song change when a song reaches its end
No tabs, 4 spaces
Grey out played/previous items
Tabs fix and better grey out album tracks
Store first/last song

Click Next several times on last song, play Prev - playback is not stuck
Don't stop playback if next/previous song cannot be played
Don't try to clean iter if another album was selected